a) Field of the Invention
This invention relates to an X-ray microscope and, more particularly, to an X-ray image forming apparatus.
b) Description of the Prior Art
Recently, researches and developments of X-ray radiation sources and X-ray optical elements have been advanced and, as one of their application systems, an X-ray microscope is proposed. As shown in FIGS. 1 to 3, various types of optical elements used in the X-ray microscope are available. FIG. 1 shows a Wolter reflecting optical system (in the figure, its reflecting surfaces are indicated only by solid lines). This is such that X rays are made incident on the reflecting surfaces at a large angle and reflected by utilizing total reflection at the reflecting surfaces, which is typical of a grazing incidence optical system. FIG. 2 shows a Fresnel zone plate utilizing diffraction. FIG. 3 shows a normal incidence type Schwarzschild optical system in which two spherical mirrors are each coated with a multilayer film. In this optical system, the multilayer film forms an artificial grating to reflect X rays by utilizing diffraction due to the grating.
Making use of the property that soft X rays cause little damage to a biological specimen, attention is also aroused as to the application to a biological microscope capable of observing the biological specimen with high resolution and with no staining. In the wavelength band of X rays having a wavelength .lambda.=43.7.about.23.6 .ANG. in particular, the absorptance of X rays in terms of carbon is high and the transmittance of X rays in terms of the molecule of water is also high, so that if it is applied to the biological microscope, the transmitted microscopic image of protein whose principal constituent atom is carbon can be observed with good contrast in water. Hence, research institutions are prosecuting researches and developments on the optical elements of X-ray multilayer film mirrors and filters, radiation sources, detectors, etc. which can be used with high accuracy in the above wavelength band.
In the wavelength band of .lambda.=43.7.about.23.6 .ANG. mentioned above, however, there is the problem of making difficult the fabrication of the optical element in which accuracy sufficient to be used as the X-ray microscope is realized and secured. Where the multilayer film reflecting mirror with high reflectance is generally designed, it is required that two kinds of substances with the largest possible difference between their refractive indices are built up alternately to form a multilayer film. With such a wavelength band, however, the refractive indices of most substances are close to unity and it is therefore difficult to choose two kinds of substances with the large difference between the refractive indices. Although a proposal is made for materials whose reflectances are expected to be somewhat improved, such as multilayer films of a structure (Ni/Sc) of laminating alternately Ni (nickel) and Sc (scandium) and another structure (Ni/Ti) of laminating alternately Ni and Ti (titanium), these materials are liable to crystallization in their evaporation, which fact makes it difficult to secure a uniform film. Additionally, in the current state-of-the-art of the film fabrication, the normal incidence mirror for the wavelength band of .lambda.= 44.about.22 .ANG. is such that a basic period of the multilayer film (the total thickness of two substances laminated in a pair) is inevitably reduced to 20 .ANG. or less, so that the fabrication of the multilayer film itself is difficult. Still further, in the wavelength band of .lambda.=43.7.about.23.6 .ANG., other problems are encountered that the high absorptance of X rays in terms of carbon makes it impossible to use organic materials as filters and narrows the range of choice of filter materials. Thus, the optical element such that practical accuracy is secured is hard of design and requires careful discussion for the choice of materials.
Even though the optical system utilizing such an optical element has been realized which can image X rays in the wavelength band of .lambda.=43.7.about.23.6 .ANG., there is the problem of making difficult the realization of a practical observing function of the X-ray microscope for observing the biological specimen with favorable contrast. The X-ray absorptance of the biological specimen depends on the density of carbon present in the specimen, the thickness of the biological specimen, and the wavelength of X rays with which the specimen is irradiated. Hence, in the case of microscopy of the biological specimen which, for example, is relatively high in carbon density and large in thickness, most of the X rays with which the specimen is irradiated are absorbed by the specimen and the resultant transmitted microscopic image is dark as a whole, poor in contrast, and hard of view, that is, diminishes the amount of information. Conversely, where the carbon density is low and the thickness is small, the transmittance of X rays is improved and the transmitted microscopic image becomes bright as a whole, but even in this case, the contrast is poor. That is, unless the above conditions of the biological specimen for determining the X-ray absorptance are all proper, the transmitted microscopic image of favorable contrast cannot be brought about.
In order to solve the foregoing problems, there is the method of adjusting the thickness of the biological specimen or the wavelength of X rays with which the specimen is irradiated in the wavelength band of .lambda. mentioned above. The former, however, uses a precision machine, such as a microtome, requiring operator skill to cut the specimen, which is disposed in the microscope optical system and irradiated with X rays, and if the resultant transmitted microscopic image lacks in contrast, the specimen will be cut again by the precision machine. Such operation is repeated through the rule of trial and error, thereby determining a proper thickness of the specimen. This has no practical use. The latter involves a wide change of design and the change of layout of the microscope optical system and the optical element in using the optical element of wave dispersion as in the zone plate or the Schwarzschild optical system, so that this method is also of little practical use and at variance with the reality.